1. Field of the Invention
The present invention relates to an optical data transmission apparatus, and more particularly relates to an optical data transmission apparatus suitable for a servo communication system in which a plurality of servo control units for controlling servo motors are connected by the multi-drop method to a full-duplex communication line connected to a system control apparatus, and the servo control unit controls the servo motor upon receiving a control instruction from the system control apparatus via the communication line, and transmits a control state thereof to the system control apparatus via the communication line.
2. Description of the Related Art
In the field of wireless data communication, a transmission signal output from a first communication unit sometimes interferes with a reception signal output from a second communication unit in communication, and a reception unit of the first communication unit is unable to satisfactorily receive the signal. In order to overcome the above drawback and achieve stable full-duplex communication, the transmission signal and the reception signal, which are electromagnetic waves, use different frequencies and different modulation/demodulation methods.
In general, the FSK (Frequency Shift Keying) method is often employed in the digital modulation method. Since the amplitude of a signal does not carry any information in the FSK method, a transmission circuit can output the signal with the maximum power at all times. Therefore, a reception circuit does not need to have an AGC circuit or the like to control a gain. A limiter circuit and a saturable amplification circuit are provided to simply amplify the reception signal.
In the FSK method, no amplitude component is necessary. Accordingly, the FSK method has a characteristic that, when a strong signal is received, a weaker signal in the same frequency is masked. In general, a weak reception signal is often an undesired interference signal. Therefore, the FSK method having high performance in eliminating a weak signal (co-channel selectivity) is often employed as a highly reliable method.
In addition to the above method, the PSK (Phase Shift Keying) method using phase modulation and the ASK (Amplitude Shift Keying) method using amplitude modulation are also available. In these modulation methods, any one of the phase, the frequency, and the amplitude of a reference carrier in a carrier frequency fc is shifted based on a baseband signal.
In order to achieve high-speed data communication, a carrier frequency fc sufficiently higher than a data frequency fs is required. Therefore, a broadband property at a high frequency is necessary.
When the generally available FSK method is employed to perform communication at a relatively low transmission rate of approximately several dozens of kbps, an inexpensive communication apparatus can be easily manufactured using demodulation ICs and peripheral components for generally available inexpensive FM radio receivers.
However, in a case where communication is required to be performed at a still higher communication rate, it is necessary to structure a complicated circuit using separate components. Further, the cost is more likely to increase due to the increase of circuit adjustment and the like. Accordingly, an inexpensive communication apparatus is limited to have a communication rate of approximately 10 Mbps in actual use.
An optical data transmission apparatus having an optical signal transmission unit and an optical signal reception unit is basically the same as the data transmission apparatus which does not use light but uses electromagnetic waves, in that the optical data transmission apparatus needs to receive signals while preventing lights from interfering with each other.
In order to solve this problem, Japanese Unexamined Patent Publication No. H11-7027 suggests a mobile unit game apparatus in which an A-channel for communication from a control apparatus to a mobile unit employs infrared light linearly polarized in a predetermined direction and a B-channel for communication from the mobile unit to the control apparatus employs infrared light linearly polarized in a direction different from the predetermined direction.
Japanese Unexamined Patent Publication No. H08-335911 suggests a bidirectional optical spatial transmission apparatus, in which a light emitting apparatus and a light receiving apparatus face each to other via a space so as to constitute a pair, and two or more pairs of the light emitting apparatus and the light receiving apparatus are arranged to face each other in the opposite positions, so that the light emitting apparatus and the light receiving apparatus in each pair can transmit/receive light to/from each other.
In the bidirectional optical spatial transmission apparatus, a laser device of the light emitting apparatus in one pair and a laser device of the light emitting apparatus in another pair are arranged and fixed such that both of the laser devices oscillate laser light beams having polarization directions substantially perpendicular to each other.
Likewise, Japanese Unexamined Patent Publication No. 2008-118542 suggests an infrared light optical signal transmission apparatus adapted to transmit a first optical signal constituted by infrared light and a second optical signal constituted by infrared light, the infrared light optical signal transmission apparatus including a first optical signal transmission unit for transmitting the first optical signal polarized in a first polarization direction and a second optical signal transmission unit for transmitting the second optical signal polarized in a second polarization direction crossed with the first polarization direction.
A relatively inexpensive LED employed as a light source of a light emitting unit has an advantage that a polarization plane of an optical signal output from the light source can be easily adjusted according to the direction of installing a polarization element.
However, unlike a laser diode, an LED is not a light source emitting coherent light. Therefore, there is a drawback that even though an optical lens is used to condense light, the LED has a poor directivity and produces a blurry light beam with the power being diverged, thus has a limited communication range. Further, there is a drawback that the LED cannot be used for high-speed communication of more than 10 Mbps due to a switching rate thereof.
On the other hand, in a case where a relatively inexpensive semiconductor laser for emitting single-mode light is employed as a light source of a light emitting unit, the semiconductor laser emits coherent light, which enables sending parallel light beams in a long distance and condensing light beams into an extremely small point by using an optical lens. Therefore, the semiconductor laser can handle high-speed communication. It should be noted that a generally available red laser diode emits single-mode light having a polarization property in either a TE mode having a polarization property parallel with a junction surface or in a TM mode having a polarization property perpendicular to the junction surface.
However, the polarization direction of the polarization element arranged at each of the light emitting side and the light receiving side needs to be aligned with the polarization direction of the light output from the laser diode. Accordingly, it is necessary to adjust the implementation direction, that is, the installation posture, of the laser diode with respect to the polarization element. Therefore, it is necessary to give careful consideration to the design of a printed circuit board and the design of a mechanism of an optical signal transmission unit including the polarization element, and there is a problem that troublesome adjustment is necessary during the assembly process.
In a case where any one of the first communication unit and the second communication unit is installed in a mobile unit, and the first communication unit and the second communication unit move relatively to each other, communication therebetween may be affected by interference, and further, an optical axis may be displaced due to vibration and the like. Therefore, a certain directivity property (area property) is required to ensure reception performance. For example, when any one of the first communication unit and the second communication unit is mounted on a rail-guided vehicle and the directivity angle thereof is small, mechanical axes of bumps and warpage of tracks (rails) are required to be highly accurate. However, it is impossible to satisfy the required accuracy under various kinds of limitations such as the cost, the operability, the secular distortion, and the like.
In particular, since the laser diode has an astigmatic difference, the position of a light emitting point appears to be different in the vertical direction and the horizontal direction of the beam. Even though an optical lens condenses light beams into parallel light, the light beam has a significant elliptic shape, and therefore, there is a problem that it is impossible to ensure a sufficient directivity property.